In the field such as radio communication, it is important that a receiver is able to receive a strong or a weak signal without overloading and be able to rapidly adjust to level variation in order to minimize the effective dead time of a network. Technique for doing this goes under the general name of Automatic Gain Control (AGC) and is already well established in the art. One of the most commonly used forms of AGC is the arrangement shown in FIG. 1.
As illustrated in FIG. 1, a gain controlled element, e.g. attenuator 10, has a signal input 11, a signal output 12 and a gain control input 13. A gain control device 14 has an input coupled to the output 12 of the attenuator 10 and an output coupled to the gain control input 13 of the attenuator 10. In operation, the gain control device 14 receives the signal outputted from the attenuator 10, generates a gain control signal based on the received signal, and then feeds back the gain control signal to the attenuator 10 via the gain control input 13, whereby the attenuator is able to adjust the gain itself on the basis of the gain control signal.
It is often that there are multiple gain controlled elements such as attenuator and amplifier in a receiving circuit. In this case, a plurality of gain control devices will be arranged for these gain controlled elements separately. On the other hand, it is possible that all these gain control devices are arranged to derive the signal having passed through these gain controlled elements at the end of the receiving circuit and feed the gain control signal back to the gain controlled elements. However, in this way, a plurality of power elements such as filter will be arranged between the gain control device and the gain controlled element. As known, each power element has its delay, the cumulative delay between the gain control device and the gain controlled element will lower AGC response speed. Although this is not a big problem for Global System for Mobile Communications (GSM) because the chip rate is low, it will be trouble for such as 20 MHz Long Term Evolution (LTE) system, where the chip rate is up to 30.72 MHz (32 ns per sample). In this case, if the AGC response time is longer than 500 ns, it means every time AGC state changes, over 15 samples data will be lost, which will impact Quality of Service (QoS) when AGC state update frequently due to complicated and fast variance signal/interference. System performance will be degraded obviously. Furthermore, lower AGC response speed will also cause a long glitch duration time.